Conventionally, there has been known an alarm which is provided on a lower surface of a ceiling of a monitoring area and generates an alarm by detecting smoke in the monitoring area. This alarm includes a casing which accommodates a circuit board or the like of an alarm circuit and a detector which detects smoke. However, in this alarm, since the detector is provided to be exposed to the outside of the casing, the alarm has a complicated appearance and thus has a possibility that the appearance of the monitoring area may be deteriorated. For this reason, in recent years, there has been a request for improving the design property of the alarm in order to improve the appearance of the monitoring area provided with the alarm.
Here, an alarm in which a detector is accommodated in a casing is proposed (for example, Patent Document 1). In the alarm, an opening is provided at a side wall of the casing, smoke is allowed to flow into the casing through the opening, and the smoke flowing into the casing is detected by the detector. Then, in the alarm, vertical and horizontal bars crossing each other are provided at the opening in order to reinforce the casing.